The present invention relates to a water purification system which utilizes an electrodeionization step and an ultraviolet radiation treatment step.
Prior to the present invention water has been purified to reduce its ion content and to reduce total organic carbon (TOC) by ion exchange treatment which effects ion concentration reduction and by exposure to ultraviolet radiation which effects oxidation of organics to form more easily removable products. Such a process is disclosed, for example, in European patent application No. 0,265,031 published Apr. 27, 1988. The system disclosed comprises a quartz tube containing an ultraviolet light through which water is poured in combination with a deionization step. In one embodiment, water is circulated sequentially through the deionization step whenever organic extractables are removed to the ultraviolet light exposure step where the organics are oxidized. It is disclosed that the 38 hours of continuous circulation through the system was required in order to reduce TOC to 1.5 parts per billion (ppb). In a second embodiment, a typical water supply system is disclosed which includes the deionization step and the ultraviolet light exposure step. In this system the water is recirculated between these two steps a plurality of times. Two alternate deionization steps are employed with one being on-line while the second is either regenerated or new ion exchange resin is added thereto. Because ion exchange resins contain organic extractables, one cannot reduce TOC to 0% concentration in a system which utilizes deionization. In addition, a batch process is required when utilizing deionization because organics are deposited on the ion exchange resin and the resin must be replaced or regenerated periodically. The addition of new resin or regeneration of depleted resin introduces an additional major problem since the new or regenerated resin contains significant quantities of organic extractables which will enter the purification system when the deionization step containing the new resins comes on- line.
Means other than ultraviolet light exposure also have been proposed to modify soluble organics in order to render them more susceptible to removal from water. Such means include exposure to sulfite, reducing pH and electrochemical oxidation. In particular, exposure to sulfites results in the formation of adducts with organics such as formaldehyde.
Electrodeionization has been proposed as a means for replacing deionization to effect ion removal from water. However, prior to the process disclosed in U.S. Pat. No. 4,632,745, no practical electrodeionization process has been available for producing substantially ion-free, high resistivity water. However, the resins utilized in electrode-ionization process also contain organic extractables.
Accordingly, it would be desirable to provide a water purification system which is capable of producing high restivity water while eliminating TOC. Furthermore, it would be desirable to provide such a process which is continuous rather than a batch process.